In the manufacture of foamed elastomeric compositions, a foamable, curable elastomeric polymer is conventionally mixed with a curative and a blowing agent which, upon exposure to elevated temperature conditions, decomposes to form gaseous decomposition products for expansion of the material. Such processes for preparing foamed elastomeric compositions (wherein the temperature of the blend is gradually raised so that foaming and curing will occur) vary greatly from processes for preparing foamed resinous or thermoplastic compositions, as such thermoplastic processes typically involve heating the resin above its softening temperature, blending in blowing agent, and letting the blend temperature gradually cool while blowing is occuring. Thus, due to such processing differences, it is not evident that a blowing agent composition suitable for thermoplastic resins would also be desirably employed for expanding elastomeric materials and visa versa.
Among conventionally employed blowing agents, azodicarbonamide has come into widespread usage as this blowing agent is non-toxic, odorless and non-flammable in character, and its decomposition products present no dangerous health hazards. However, azodicarbonamide has the disadvantage that its decomposition temperature is relatively high, 210.degree. C., rendering it difficult to use with many polymeric materials--particularly those which are susceptible to thermal degradation or deterioration at such relatively high temperatures. Consequently, when used with polymeric materials having such disadvantageous thermal characteristics, it is desirable to add a decomposition accelerator (activator) to lower the decomposition temperature of the azodicarbonamide blowing agent.
Although a large number of such activators for lowering the decomposition temperature of azodicarbonamide have been disclosed, the performance requirements of blowing agents have now become more and more critical and changes in the composition of the blowing agents can have significant effects on the end product.
Among the activators for azodicarbonamide which have been disclosed in the past are various zinc salts, which are most typically employed in conjunction with a third component.
Thus, Japanese Patent Publication No. 1978-145876 of Shimagana et al describes a foamed thermoplastic vinyl chloride composition which is produced employing a blowing agent comprising azodicarbonamide and one or more zinc salts of organic carboxylic acids having at most six carbon atoms. It is noteworthy that the compositions actually exemplified by this patent all additionally contain barium stearate which, although nominally employed as a heat stabilizer, will have considerable effect upon the activation of the azodicarbonamide. Accordingly, the compositions of Japanese Publication No. 1978-145876 are processed at up to 180.degree. C., and are expanded at 200.degree. C.
Somewhat similarly, Japanese Patent Publication No. 1980-133433 to Tomimoto et al discloses a blowing agent composition comprised of (i) azodicarbonamide; and (ii) the reaction product of an amine with a zinc compound. A broad range of zinc compounds (including zinc chloride, zinc bromide, zinc iodide, zinc sulfate, zinc sulfite, zinc carbonate, zinc nitrate, zinc formate, and zinc acetate) and of amines (including ammonia, hydrazine, hydroxylamine, methylamine, ethylamine, isopropylamine, n-butylamine, n-hexylamine, n-dodecylamine, n-octadecylamine, ethylenediamine, propylenediamine, hexamethylenediamine, monoethanolamine, aniline, guanidine, and aminoguanidine) which may be co-reacted is disclosed. However, it is to be noted that Tomimoto et al indicate that if one simply adds a mixture of unreacted zinc compound and amine to azodicarbonamide, the full desired effect of the invention cannot be realized.
In addition, U.S. Pat. Nos. 4,444,679 and 4,472,536 and 4,482,650, all to Rowland et al, described three-component blowing agents comprised of (i) azodicarbonamide; (ii) zinc oxide, zinc carbonate or a zinc salt of a C.sub.1 -C.sub.4 organic acid; and (iii) an aromatic sulfinic acid or a metal salt of an aromatic sulfinic acid.
Other activators for azodicarbonamide are disclosed in U.S. Pat. No. 4,312,776 to Puri et al (which shows a blowing agent composition comprised of (i) azodicarbonamide; (ii) a chromium sulfate; and (iii) a zinc salt or oxide); in U.S. Pat. No. 3,846,350 to Schentger (which shows the expansion of polychloroprene employing a blowing agent composition comprised of azodicarbonamide and at least one metal oxide); and in U.S. Pat. No. 4,554,294 to Hunter et al (which shows blowing agents comprised of a mixture of azodicarbonamide and the zinc salt of nitro-urea).
It has also been disclosed in the past that, for certain applications, zinc formate may function as a desirable blowing agent. Thus, British Pat. No. 1,567,417 shows foamed thermoplastic compositions which are produced by heating a blend of polyester and zinc formate to the 280.degree. C. decomposition temperature of zinc formate (in the sole Example, a Wood's metal bath heated to 240.degree. C. is employed). Similarly, German Offenlegungsschrift No. 2,148,099 describes the expansion of thermoplastic polymers using neutral metal salts of formic and/or oxalic acid optionally with a stabilizer such as sodium phosphite, while U.S. Pat. No. 4,438,223 (to Hunter) describes a blowing agent composition comprised of (a) zinc formate; and (b) certain sodium or potassium salts. Similarly, Dollimore et al in Journal Of Inorganic Nuclear Chemistry, 29, 621-627 (1967) describe the decomposition behavior of zinc and manganous formates while Djega-Mariadassou et al in Bull. Soc. Chim. France, 9, 3166-3173 (1971), "Study of Decompositions Causing Simultaneous Release of Several Gases, IV--Thermal Decomposition Of Zinc Formate Dihydrate and Morphology of Zinc Oxide Residual" detail the decomposition behavior of zinc formate and the role of zinc oxide formed during its decomposition.
While the various blowing agent compositions described above are suitable for certain uses, there are nevertheless restrictions imposed upon the use of all blowing agents which arise from the processing limitations of the polymers to be expanded. Thus, a polymer with a degradation temperature of 200.degree. C. could not be expanded using a blowing agent with an activation temperature of 250.degree. C. Consequently, there is a need for new processes to expand certain polymers which may have relatively narrow preferred processing temperatures and conditions.
Accordingly, it is an object of this invention to provide a process for expanding elastomeric materials having a processing temperature of between about 130.degree. C. and about 170.degree. C.
It is another object of this invention to provide a novel expanded elastomeric composition.
These objects and other additional objects will become more fully apparent from the following description and accompanying Example.